Bacterial Slime Cities
A Bacterial Slime City
Above: The biofilm life-cycle. Isolated cells adhere to the substrate (1) and move across the substrate until
they aggregate into groups. Once aggregated they secrete slime to form a small microcolony (2) and undergo
changes in gene expression, which suppresses flagellin synthesis and causes other changes to produce a
phenotype more suited to biofilm ‘city’ life. These microcolonies eventually grow upwards from the substrate as
mushroom-shaped or cylindrical columns (3). These larger microcolonies are exposed to higher fluid flows
above the lower boundary layer and this flow may be conducted through water channels in the base of the
biofilm (blue arrow). Some cells begin to differentiate into flagellated swarmer cells which are released high
above the substrate when the columns rupture (5). Columns may undergo several cycles of rupture and
swarmer dispersal, regrowth, and rupture and swarmer dispersal. The flagellated swarmer or planktonic cells
(6) are dispersed both passively by fluid flow and actively, and can even swim upstream (7) as discussed in the
text. Swarmer cells may undergo several stages of cell-division in the planktonic stage, but eventually adhere to
the substrate to complete the cycle (8). Additional mechanisms of cell dispersal from biofilms are discussed in
the text. (Note that this image is copyrighted and permission must be sought before using it elsewhere).
Many bacteria are now known to form multicellular structures and possibly all bacteria are capable of this. The majority of
in animals, plants, some algae and fungi and indeed, they are rarely physically connected directly. In this way a biofilm is
not like a multicellular organism, but it is a multicellular colony of a less well-organised type. The bacteria are connected
by a mass of slime which they secrete and in which they are embedded. This slime forms the bulk of the biofilm. We can
think of a biofilm as a 'slime city'. Although a single species is capable of biofilm formation, in nature biofilms are often
multi-species and eukaryotic cells such as yeasts may also be incorporated.

A biofilm is founded when one or more planktonic cells adheres to a suitable substrate. The planktonic cells are the single
cells we normally think of as bacteria. In many species the planktonic cells are flagellated and so capable of swimming and
then they are also called swarmer cells. The adherent cells secrete slime and multiply by binary fission to form a small
microcolony. The biofilm consists of a surface sheet of slime from which emerge various columns or tower-like structures
that vary in form. As nutrients become used-up, the cells in the centre of these columns become deprived of nutrients and
undergo changes - they transform into swarmer cells, the column ruptures and the swarmer cells escape.

What is the function of biofilms?

The swarmer cells are essentially functioning as spores. Biofilm columns are typically 300-400 micrometres tall and this
height is sufficient to break the boundary layer of stagnant fluid that develops over surfaces in typical conditions. This
allows the bacteria to access more oxygen and nutrients and also aids swarmer cell dispersion. In this way, bacteria can
more efficiently utilise a food source and then colonise new areas when this food source becomes exhausted. Biofilms also
facilitate adhesion or attachment of cells to the surface and protect them from many agents by the secretion of the
copious slime. The biofilm structure probably also confers some protection against grazing protozoa and filter feeders
which readily ingest planktonic cells (e.g. rotifers, vorticella, sponges). In nature, many bacteria succumb to predation, and
defences that work against grazing amoebae may also work against mammalian white blood cells (albeit with various
modifications) and so biofilm formation is of great importance to many infectious bacteria. The formation of biofilms in the
body (by commensals and pathogens) greatly increases the resistance of these bacteria to both the immune system and
antibiotics.

Other types of multicellularity in bacteria

Many bacteria form filaments, in which the cells are connected end-to-end in a chain. These chains forms when cells fail to
separate after binary fission and as the cells divide the chains lengthen. The cells may be joined at their end-walls or by a
common slime sheath. This form of multicellularity reaches its greatest complexity in the cyanobacteria (blue-green
bacteria). These bacteria are photosynthetic and the chains may be enclosed in larger colonial structures - they form
easily visible filamentous strands and nets, where each fibre is many cellular filaments together, or they may form
spherical slime-filled capsules, 1-2 cm in diameter and they may even form reef-like structures called stromatolites which
were abundant in prehistoric times, in the Age of the Prokaryotes when bacteria were the dominant life-form on Earth, and
are still frequent in the Dead Sea which is too salty for many other forms of life. In stromatolites, generations of
cyanobacterial biofilms trap sand grains and the like, cementing them together to build stony column-like or
mushroom-shaped structures which may reach several metres in height and diameter - quite a feat of construction for a
microbe!

A Pov-Ray 3D computer model of a filamentous cyanobacterium, of the
Anabaena type, is shown below:
Anabaena
Anabaena
Anabaena gif
The filaments of some cyanobacterial species show differentiation - some cells are specialised for certain functions. In
the filament above, their two enlarged cells, the leftmost one is a
heterocyst, which is specialised for nitrogen-fixation,
and the one on the left is an
akinete - a dormant cell with a thickened wall that is resistant to harsh conditions.

In some cyanobacteria individual filaments live inside slime-tubes which they secrete and many tubes may be arrayed
together. The filaments can glide up and down inside their tubes and this enables the bacteria to adjust their height
above the surface to reach the light and oxygen and other materials they may need. Thus, being filamentous is an
advantage since again it allows bacteria to break free of the stagnant boundary layer and utilise resources in the
free-flowing water. Many cyanobacterial filaments can glide over the surface, enabling them to move up structures
projecting above the boundary layer.

Some filamentous bacteria form net-like biofilms which float in the water column.
Thiovulum majus forms floating veils,
with the cells held together by slime threads, and the veils are ventilated by the many beating flagella of the
constituent cells. These nets can be moved up or down in the water column to optimise their position.

In some filamentous bacteria the cells are connected by electrical conjunctions and an electrical signal (such as the
flow of protons) can pass from cell to cell. This enables them to coordinate their gliding activity, so that they move
forwards or backwards in concert for maximum locomotive efficiency. This is the closest bacteria seem to get to
forming true multicellular tissues, even if these filaments are rather one-dimensional.

Rosettes and Hollow Balls

Some bacteria, like Caulobacter, have stalks tipped in adhesive holdfasts and neighbouring cells may adhere to
one-another to form a rosette. A species of motile and magnetotactic bacterium has been found forming a hollow ball
of about 14-20 conical cells.

Download a pdf for more information about bacterial movement and filament formation:
Prokaryotes_motility
Above: stages in the development of multicellular fruiting bodies, from left to right, in a myxobacterium. Myxobacteria are
soil organisms that are usually found through the development of their fruiting (sporing) bodies on tree bark,
decomposing plant remains and animal dung. Most are
bacteriolytic, killing other bacteria by secreting antibiotics and
then breaking down their cells with secreted extracellular enzymes (e.g. proteases to break-down proteins, nucleases to
break-down DNA and RNA and lipases to break-down lipids and then absorbing the nutrients (such as short-chains of
amino acids, the remnants of digested proteins) released from the ruptured (lysed) bacterial cells. Some are
algicidal,
killing cyanobacteria and eukaryotic algae. Others are
cellulolytic, secreting cellulase enzymes to degrade cellulose (a
component of
plant cell walls) into sugars which are absorbed for nutrition. The individual bacteria may be rod-shaped or
spherical, depending on species (and possibly growth conditions?). in Myxococcus, the vegetative cells are elongated
rods, but they round-up to form spherical spores (myxospores). The fruiting bodies of most species are coloured,
glistening droplets filled with myxospores, which may be spherical or rod-shaped.

Bacteria that live in slime cities

The picture above shows part of a bacterial biofilm. This is a sheet of bacterial cells encased in slime. The bacterial cells
may be tightly packed into groups (forming organised 'tissues' in which cells lie side-by-side as in some
myxobacteria) or
they may be loosely dispersed within the slime. The edges of the slimy mass are often motile and slowly advance over the
surface, consuming any suitable food items that are in their path, which in the case of the predatory myxobacteria are
other bacteria. Most bacterial biofilms are composed of rather loose slime which is easily fragmented when handled and
so most of these structures escaped scientific discovery for a surprisingly long time and were only discovered a few years
ago. Others, like those of the myxobacteria, are firmer and more rigid and have been known for decades. It is now known
that many (if not the majority) of bacteria that have been investigated have a multicellular slime stage, called a biofilm or
'slime city'. The term 'biofilm' commonly refers to the looser aggregates of cells embedded in slime, which is by far the
most common form of multicullar structure formed by bacteria. Prior to these recent discoveries it was thought that most
bacteria spent their lives as single cells. Now it is known that bacteria alternate between single cells that disperse to new
habitats, and are often highly motile (so called
swarmer cells, which may have flagella) and slime colonies that colonise
solid surfaces and the surface of stagnant water.

There are several advantages for bacteria working together in multicellular slime cities, first they are more resistant to
chemical attack, for example, by antibiotics produced by other micro-organisms to kill their rivals. Secondly, the bacteria
can form towering structures (usually no more than one a millimetre in height, but sometimes several centimetres tall) that
enable them to lift themselves up from the stagnant water near the surface, into the flowing water above, in order to tap
into the nutrients and oxygen carried by the water. It also allows myxobacteria to hunt other bacteria in packs,
surrounding their prey, dissolving it and then absorbing it. Thirdly, it enables them to release
spores or swarmer cells
from the tops of these towers, into the water stream, where they can be carried to new habitats for colonisation. When a
swarmer cell finds a suitable surface, it will attach and maybe form a new biofilm. Otherwise, swarmer cells can live and
reproduce happily in the water column or soil as single-celled organisms. The picture above shows several stalked
colonies that will give rise to spores or swarmer cells. These spore-producing '
sporangia' (singular sporangium) come in a
tremendous variety of forms, depending upon species, some are stalk-less domes, others are spheres on top of stalks,
and some form complex branching tree-like structures. Eventually the swollen ends of these sporangia will burst open to
release the spores or swarmer cells.
slime city

Young, K.D., 2006. The Selective Value of Bacterial Shape. Microbiol. Mol. Biol. Rev. 70:660-703.

Skilbeck
et al. Quorum-sensing and multiple antibiotic resistance